A HVAC system can have a chiller that typically includes a compressor, heat exchangers such as a condenser, an evaporator, and an expansion device forming a refrigeration circuit. Refrigerant vapor is generally compressed by the compressor, and then condensed into liquid refrigerant in the condenser. The liquid refrigerant is then expanded by the expansion device (e.g. expansion valve) to become low-pressure low-temperature two-phase refrigerant and is directed into the evaporator; and the two-phase refrigerant can then exchange heat with a process fluid, such as water, in the evaporator. The two-phase refrigerant may be vaporized in the evaporator and return to the compressor.
In a chiller, the condenser and/or the evaporator can be a tube-and-shell type heat exchanger. The condenser and/or the evaporator can maintain certain levels of liquid refrigerant in the shell in operation. Maintaining a proper level of liquid refrigerant in the condenser and/or the evaporator may help increase operational efficiency of the chiller.